EP3425997A2

EP3425997A2 - Carbon heating element and manufacturing method for the same

Info

Publication number: EP3425997A2
Application number: EP18174315.4A
Authority: EP
Inventors: Youngjun Lee; Kap Seung Yang; Sang Wan Kim
Original assignee: LG Electronics Inc; Industry Foundation of Chonnam National University
Current assignee: LG Electronics Inc; Industry Foundation of Chonnam National University
Priority date: 2017-05-26
Filing date: 2018-05-25
Publication date: 2019-01-09
Anticipated expiration: 2038-05-25
Also published as: CN108934087A; US11096249B2; US20180343704A1; CN108934087B; EP3425997A3; EP3425997B1; KR20180129446A; KR102004035B1

Abstract

The present invention, which aims to efficiently dissipate heat and prevent disconnection or destruction of a heating element to prolong a lifespan thereof without generating a spark and plasma under a high voltage, relates to a heating including carbon (C) and silicon carbide (SiC), and the heating element characterized by having a thermal conductivity of 1.6 W/m·K or more.

Description

A carbon heating element that is a heat source of a carbon heater used in the field of a cooking appliance such as an oven is disclosed herein.
In recent times, an over using a heater as a cooking appliance for family use or commercial use has been widely used.
FIG. 1 is a perspective view showing a general structure of an oven.
Referring to FIG. 1, an oven 1 is provided with a cavity 2 in which food to be cooked is placed, a door 3 for selectively opening the cavity 2, and a heater 6 for applying heat to the cavity 2.
In particular, the heater 6 is provided with one or more heating elements, and is protected by a cover 8 from the exterior of the cavity 2.
Further, in order to apply an electromagnetic wave heating method, a magnetron 4 is provided on the exterior of an upper surface of the cavity 2. The magnetron 4 generates electromagnetic waves, and the generated electromagnetic waves are radiated to an inner space of the cavity 2 through a predetermined waveguide and a stirrer.
In addition, a sheath heater 5 is provided at an upper side of the inner space of the cavity.
A carbon heater among various kinds of heaters, which uses a radiant heating method, is mainly used for the sheath heater 5 and the heater 6.
Conventionally, a carbon fiber made of a fibrous carbon material having a carbon content of 90% or more is mainly used as a heating element of the carbon heater.
Since the carbon fiber is made of a carbon material, it has a property of carbon itself, which absorbs microwave.
Further, the carbon fiber has another inherent property that a ratio of a fiber length to a fiber diameter is very large in view of a shape of "fiber".
These inherent properties of such carbon fiber cause some problems when the carbon fiber is used as a heating source of the oven.
FIG. 2 schematically shows a conventional carbon heater 10, and FIG. 3 shows each carbon filament constituting the carbon fiber in the assembly.
Referring to FIG. 2, the conventional carbon heater 10 includes a tube 11 made of a quartz material, a carbon fiber 13, and a metal wire 15 connected to each of opposite ends of the carbon fiber 13.
Here, an outer electrode 17 is electrically connected to the metal wire 15 so as to be exposed to the outside of the tube 11 through the opposite ends of the tube 11.
The quartz tube 11 has a sealed interior, and is filled with vacuum or inert gas so that the carbon fiber 13 arranged therein is not oxidized at a high temperature (e.g., 1,000 to 1,200°C)
As shown in FIG. 3, the carbon fiber of FIG. 2 is made of individual carbon filament. However, the filaments have not only a diameter of several micrometers (µm) but also an interval between filaments of several micrometers (µm).
Thus, when a voltage is applied between the outer electrodes, the voltage is also applied between the filaments within a very narrow internal distance, and as a result, a very high voltage per unit length is applied between the filaments. For example, when a external voltage of 10 V is applied to an interval of 1 µm, a high voltage of about 10⁷ V/m is applied between the filaments.
In this case, a local high voltage applied to the filaments is likely to generate a dielectric breakdown and a spark.
Also, when a high voltage is applied between the filaments, plasma is very likely to occur due to an inert gas atmosphere under a high voltage, even though a dielectric breakdown or a spark does not occur in the filaments.
Conventionally, a shield member was provided between a carbon heater and a cabin to suppress the reaction of the plasma and the like, and the progress of plasma light to the cabin.
However, since the shield member not only shields the plasma light, but also partially blocks radiation light emitted from the carbon heater, the radiation efficiency of the oven is greatly lowered.
Hence, there is a growing demand for a new type of carbon heating element, not a conventional carbon fiber, as the heating element of the carbon heater.
The related art is disclosed in KR Patent Application Publication No. 10-2011-0109697 (October 6, 2011 ).
The present invention aims to provide a new carbon heater in which a dielectric breakdown, a spark, and plasma do not occur even under a high voltage.
Further, the present invention aims to provide a heating element for a new carbon heater which does not generate plasma even under a high voltage and encapsulation gas in the carbon heater.
According one aspect of the present invention, a heating element, which is capable of efficiently dissipating heat and preventing disconnection or destruction of the heating element to prolong a lifespan thereof without generating a spark and plasma under a high voltage, may include carbon and silicon carbide (SiC), and the heating element may have a thermal conductivity of 1.6 W/m·K or more.
Preferably, the SiC may include β-SiC and α-SiC.
Preferably, the degree of crystallization of SiC included in the heating element may have a full width at half maximum (FWHM) value of 0.14 or more as a result of analyzing an X-ray diffraction (XRD) pattern.
Preferably, the heating element may include silicon oxide (SiO₂).
In particular, the total amount of oxygen in the heating element may be less than 2 wt.% (hereinafter referred to as "%" or "wt.%").
Preferably, the heating element may have a maximum surface temperature of 1,100 °C or less.
Preferably, the heating element may have a specific resistance of (11∼16)*10^-2Ωcm. According to another aspect of the present invention, the heating element, which has a relatively small surface area to achieve excellence in surface oxidation and surface erosion resistance at a high temperature, may be solid.
Also, the heating element may be hollow.
According to the present invention, a method for manufacturing a heating element in a carbon heater, which is capable of efficiently dissipating heat and preventing disconnection or destruction of the heating element to prolong a lifespan thereof without generating a spark and plasma under a high voltage, may include a mixing process to mix component materials of the heating element; a thermal extrusion process to form a shape of the heating element by extruding and injecting the mixed materials; a stabilization heat treatment process to form a coupling structure of carbon and oxygen of the binder within the heating element; and a carbonization heat treatment process to out-gas a volatile component out of the components constituting the composition of the heating element and carbonize the remaining components.
Preferably, the component materials may include a base material determining the specific resistance of the heating element, the base material including SiC; a specific resistance controlling agent for controlling the specific resistance of the heating element, the specific resistance controlling agent including SiO₂; a lubricant including graphite; and a binder for mechanical coupling between inorganic powders, the binder including a novolac resin.
Preferably, the extrusion process may be performed at a speed of about 60rpm at 100 to 200□.
Preferably, the stabilization heat treatment process may be performed at 270 to 320°C for 10 minutes to 2 hours.
Preferably, the carbonization heat treatment process may include a first carbonization heat treatment process of out-gassing at 600 to 1,000°C for 10 minutes to 2 hours.
Preferably, the carbonization heat treatment process may include a second carbonization heat treatment process and/or a third carbonization heat treatment process.
Here, the second carbonization heat treatment process may be performed at 1,200 to 1,400°C for 10 minutes to 4 hours, and the third carbonization heat treatment process may be performed at 1,500 to 1,700°C for 10 minutes to 4 hours.
Unlike a carbon heater using a conventional carbon fiber, the carbon heating element of the present invention does not generate a local voltage concentration between the filaments, which is a disadvantage inherent in a fiber shape, thereby fundamentally preventing a dielectric breakdown or a spark from occurring.
Further, unlike the carbon heater using the conventional carbon fiber, the carbon heating element of the present invention may fundamentally prevent plasma from occurring due to a local high voltage, and may improve a decline in the radiation efficiency because the shield member is not necessary.
Furthermore, the carbon heating element of the present invention uses the binder made of the powders and the resin as a starting material, and thereby it is possible to easily manufacture a carbon heater having a desired shape necessary for an oven having various sizes and shapes.
Also, the carbon heating element of the present invention may control the specific resistance and power of the carbon heater by changing components and composition ranges of the composition, and thereby it is possible to improve the degree of freedom of the electrical design of the carbon heater.
In addition, the carbon heating element of the present invention has excellent thermal conductivity, so that heat can be efficiently dissipated in the vicinity of a terminal portion to which external power is supplied. As a result, it is possible to prevent a breakage or disconnection of the heating element in the vicinity of the terminal portion of the carbon heater, thereby improving a service life.
Meanwhile, the carbon heating element of the present invention has a relatively small surface area ratio in comparison to the conventional carbon fiber, thereby achieving excellence in resistance to surface oxidation or surface erosion which may occur frequently at a high temperature. Also, such property of the composition makes it possible to omit a post treatment process such as a surface coating and the like which is necessary for the conventional carbon fiber, thereby improving lead time and productivity.

FIG. 1 is a perspective view showing a general structure of an electric oven.
FIG. 2 schematically shows a configuration of a conventional carbon fiber assembly.
FIG. 3 is an enlarged view of a carbon fiber of FIG. 2.
FIG. 4 is a flow chart schematically showing a method for manufacturing a carbon heating element using a carbon composite composition.
FIG. 5 schematically shows a carbon heater.
FIG. 6 shows electrical conductivity of a carbon heating element according to third carbonization heat treatment temperature.
FIG. 7 shows specific resistance and power of a carbon heating element of the present invention according to third carbonization heat treatment temperature.
FIG. 8 shows thermal conductivity of a carbon heating element according to third carbonization heat treatment temperature.
FIG. 9 shows the temperature stable regions of main crystal polymorphs made of a SiC material.
FIG. 10 shows XRD patterns of SiC and SiO₂ at different sintering temperatures.
FIG. 11 is a photograph showing destruction of a carbon heating element having low thermal conductivity.
FIG. 12 shows an XRD pattern of a carbon heating element, and FWHM measurement results according to third carbonization heat treatment temperature.
FIG. 13 shows SEM-EDS analysis results of components of a carbon heating element before and after performing a third carbonization heat treatment.
FIG. 14 shows power and surface temperature of a carbon heating element according to specific resistance.
FIG. 15 shows yield measurement results according to temperature of a third carbonization heat treatment performed on carbon heating element compositions.
FIG. 16 shows shapes of a solid carbon heating element.
FIG. 17 shows a carbon heating element formed in a tube shape having a central aperture.
FIG. 18 shows a carbon heating element having a shape in which a portion of a tube having a central aperture is cut such that a circular arc is provided with an opening.

Hereinafter, exemplary embodiments of the present invention will be described in detailed with reference to the accompanying drawings such that those skilled in the art can easily carry out the invention. The present invention is not limited to the exemplary embodiments disclosed herein but may be implemented in various different forms.
In order to clearly describe the present invention, the description irrelevant to the present invention has been omitted. Same or like reference numerals designate same or like elements throughout the specification. Further, some exemplary embodiments of the present invention will be described in detail with reference to the illustrative drawings. Regarding the reference numerals assigned to the elements in the drawings, it should be noted that the same elements will be designated by the same reference numerals, wherever possible, even though they are shown in different drawings. Furthermore, in the description of the exemplary embodiments, the detailed description of well-known related configurations or functions will be omitted when it is deemed that such description will cause ambiguous interpretation of the present invention.
Also, in the description of the elements, terms such as first, second, A, B, (a), (b) or the like may be used herein when describing elements of the present invention. Each of these terms is not used to define an essence, order or sequence of a corresponding element but used merely to distinguish the corresponding element from other element(s). It should be noted that if it is described in the specification that one element is "connected," "coupled" or "joined" to another element, the former may be directly "connected," "coupled," and "joined" to the latter or "connected", "coupled", and "joined" to the latter via another element.
In addition, in the implementation of the present invention, the features of the present invention may be described as being performed by separate elements for convenience of illustration. However, these features may be implemented by a single device or module or one feature may be implemented by several devices or modules.
Hereinafter, the carbon heating element and the method for manufacturing the same according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.
Manufacturing of the carbon heating element starts with preparing a heating element composition including an inorganic powder capable of serving as the main component of a heating element to be used as a high temperature heater and a binder for coupling the powder particles to one another, as essential components.

Firstly, the inorganic power may include various inorganic components as shown in Table 1 below.

	Silicon Carbide (SiC)	Silicon Oxide (SiO₂)	Aluminium Oxide (Al₂O₃)	Zirconium Oxide (ZrO₂)	Boron Nitride (BN)	Molybdenum Silicide (MOSI)
Melting point (°C)	2,730	1,600 °C	2,072 °C	2,715 °C	2,973 °C	2,030 °C
Specific resistance (Ωcm)	> 10⁸	> 10¹⁴	> 10¹⁴	> 10⁴	> 10¹³	2 * 10^-5
Thermal conductivity (W/m·K)	41	1.5	35	2.7	20	25

The composition of a starting material for manufacturing a final carbon heating element, is characterized in that at least one of the inorganic powders is included.
In particular, SiC is the most preferable because it may stably maintain specific resistance and electric conductivity which are necessary properties for the heating element. Further, in case that the carbon composite composition is formed without SiC, the specific resistance is so high that the heater made of such composition may not be used as a heater.
In the meantime, ZrO₂ and MOSI each have a disadvantage in that the specific resistance is so low that the heater made of such composition may not be used as a heating element, but they may be added to control the specific resistance of a heating element including another component as a main component.
Conversely, SiO₂ and Al₂O₃ are added as a specific resistance controlling agent for controlling the specific resistance of a finally manufactured carbon heater since they each inherently have high specific resistance.
Here, SiC is preferably added in an amount of 50 to 75% out of the total weight of the composition, for the following reasons.
When an amount of SiC is less than 50%, the specific resistance of the finally manufactured carbon heater is excessively high and the thermal conductivity thereof is lowered, and thus disconnection is likely to occur. When the amount of SiC exceeds 75%, the specific resistance of the carbon heater is so low that the heater made of such composition may not be used as a heater.
Meanwhile, SiO₂ is preferably added in a maximum amount of 24% of the total weight of the composition, for the following reasons. When an amount of SiO₂ exceeds 24%, the thermal conductivity of the carbon heater is drastically lowered, and thus a terminal disconnection may occur. Also, when the manufactured carbon heater has excessively high specific resistance due to high specific resistance inherent to SiO₂, an additional design change such as reducing the length of the heater or widening the cross-sectional area thereof may be required.
Further, it is possible to make a specific resistance controlling agent by adding Al₂O₃ in addition to SiO₂ or only with Al₂O₃. Here, even when the specific resistance controlling agent is made of only Al₂O₃, its amount is limited to a maximum of 24% for the same reason as SiO₂.
Next, the composition of a starting material for manufacturing a final carbon heating element includes an organic resin as a binder.
A binder is a component which is added for mechanical coupling (adhesion) between the powders at a relatively low temperature before the inorganic powders serving as a heating element of the carbon heater are coupled to one another by diffusion or melting at a high temperature.
The binder also performs a function of supplying carbon which is a main component of the carbon heater which is a final product.
Out of candidate materials for the binder, a novolac resin, which is a type of phenolic resin and has excellent heat resistance, is used. The novolac resin is one of the phenolic resins produced by the reaction of phenol and formaldehyde, and is generally produced when a catalyst is an acid.
However, the binder is not limited to the phenolic resin, particularly the novolac resin. Specifically, a resol resin among phenolic resins may be used as the binder. In addition to the phenolic resin, an organic resin such as an acrylic resin also may be used as a binder which generally serves an adhesive function.
The resin used as the binder is preferably added in an amount of 15 to 30% out of the total weight of the composition.
When an amount of resin is less than 15%, not only an extrudate obtained by performing a post extrusion process is easily broken but also an amount of carbon in the finally formed carbon heating element is too low, so that the specific resistance of the carbon heating element becomes too higher in comparison to the specification thereof.
When an amount of resin exceeds 30%, the extrudate after the extrusion process has poor stability in terms of a shape, and the final carbon heating element is more likely to have a dimensional defect, and an amount of carbon in the final carbon heating element becomes higher, whereby the specific resistance of the carbon heating element is too lower in comparison to a required level of specific resistance.
Also, the composition of the carbon heating element includes a lubricant in order to reduce the friction between the composition materials and a die during the extrusion process. In the present invention, the final product is a carbon heater, and thus it is preferable to include carbon as a lubricant component.
Graphite, carbon black, and activated carbon may be used as the lubricant. In particular, graphite, which is a most widely used lubricant, has excellent lubrication properties during the extrusion process.
As it pertains to this matter, the present inventors have identified that the graphite performs not only a function as a lubricant but also a function as a curing agent for the novolac resin.
In general, it is known that the novolac resin is not cured by itself. In particular, it is known that a curing agent called "hexamine" is separately required for thermal curing of the novolac resin.
Further, it was identified that, when the graphite is included in the carbon composite composition, the carbon composite composition is cured even without the curing agent after the extrusion process is performed thereon. But, the mechanism thereof has not been identified, yet.
Even when the novolac resin is used, the curing agent such as hexamine may be added to the composition of the carbon heating element.
Conversely, when the resol resin among other phenol resins is used as a binder, the curing agent is unnecessary because the resol resin generally may be thermally cured by itself without the curing agent.
In addition, when other resins such as acrylic resin and the like are used as a binder, the binder may be cured by using the curing agent or by using thermal curing or photo curing, if necessary. When the photo curing is used, a photoinitiator may be additionally included. Moreover, various additives also may be included.
The graphite is preferably added in an amount of 0.1 to 10% out of the total weight of the composition.
When an amount of graphite is less than 0.1%, the friction between the composition materials and the die increases during the extrusion process, and after the extrusion process, the curing is insufficient and the extrudate has poor stability in terms of a shape, and thereby the final carbon heater is more likely to have a dimensional defect.
Conversely when the amount of graphite exceeds 10%, the curing reaction proceeds too fast during the extrusion process, which makes processing such as extruding difficult, and the amount of carbon in the final carbon heating element becomes higher, and thereby the specific resistance of the carbon heating element is lower in comparison to a required level of specific resistance.
In the following, a carbon heater manufacturing method using the above-described carbon heating element composition will be described.
A method used when manufacturing another functional material of the same composition may be applied to the carbon heater manufacturing method.
More specifically, as shown in FIG. 4, the manufacturing method starts with a process of mixing an inorganic power and a phenolic resin binder (S 100).
In the mixing process, raw materials each having desired components and composition ranges are sufficiently mixed for a desired time using an apparatus such as an attrition mill.
Next, the mixed raw materials are thermally extruded using a general extruder widely used in the field of polymer injection so as to shape the carbon heating element (S 200). An extrusion condition may be a speed of 60 rpm at 100 to 200°C, but is not limited thereto. The extrusion condition may be changed according to the components and the composition ranges of the inorganic powder and the binder.
Also, an injection process using a mold instead of the extrusion process may be used.
The shaped carbon heating element is subjected to a stabilization heat treatment process at a high temperature (S 300).
The stabilizing heat treatment process (S 300) is a heat treatment process for inducing a coupling structure of carbon and oxygen of the binder. The binder is cured such that the carbon composite composition extruded as a result of the stabilization heat treatment process maintains its extruded shape to secure mechanical stability.
The stabilization heat treatment process was performed at 270 to 320°C for 10 minutes to 2 hours in the atmosphere.
When the stabilizing heat treatment process is performed at a temperature lower than 270 °C, it is impossible to secure the curing of the binder. Conversely, although the upper limit of the stabilization heat treatment temperature is not technically limited, it is preferable not to raise the temperature to an excessively high temperature, in terms of energy efficiency.
Next, the cured composition is subjected to a carbonization heat treatment process (S 400).
The carbonization heat treatment process aims to produce an active component of the carbon heater that is a final product by out-gassing a volatile component among the components constituting the composition and carbonizing the remaining components.
The carbonization heat treatment process is divided into three steps.
A first carbonization heat treatment process is performed at a relatively low temperature of 600-1,000°C for 10 minutes to 2 hours in an inert gas atmosphere such as nitrogen in comparison to a subsequent second carbonization heat treatment process.
The first carbonization heat treatment process mainly aims to volatilize components other than carbon among binder components, and components other than carbon which may exist in impurities and the like included in components other than the binder components among components of the composition.
A second carbonization heat treatment process is performed, immediately after the first carbonization heat treatment process. The second carbonization heat treatment process is performed at a temperature of 1,200 to 1,400°C for 10 minutes to 4 hours in the inert gas atmosphere such as nitrogen to carbonize the remaining components of the carbon composite composition after the out-gassing step.
When the temperature of the second carbonization heat treatment process is lower than 1,200 °C, the components are incompletely carbonized, and thus the heating element of a carbon heater has a lower electrical conductivity.
Conversely, when the temperature of the second carbonization heat treatment process is higher than 1,400 °C, the vaporization of the "-CC-" structure resulting from a binder material or the like occurs too many times, and thus the yield of the heating element of the carbon heater is greatly lowered.
In order to improve productivity, the first and second carbonization heat treatment processes may be integrated and operated in a single carbonization heat treatment process.
Also, the carbonization heat treatment process may include a separate third carbonization heat treatment process to adjust or improve the mechanical and/or electrical properties of the carbon heater after performing the second carbonization heat treatment process.
The third carbonization heat treatment process is performed at a temperature of 1,500 to 1,700 °C for 10 minutes to 4 hours in the inert gas atmosphere such as nitrogen.
When the temperature of the third carbonization heat treatment process is lower than 1,500 °C, the carbon heating element may be disconnected due to low thermal conductivity.
Conversely, when the temperature of the third carbonization heat treatment process is higher than 1,700 C, SiC has a higher degree of crystallization. As a result, the carbon heating element has an excessively lowered specific resistance.
The present inventors have identified that the physical properties of the final carbon heating element may be adjusted according to the components and the composition ranges of the components for a carbon heating element.
Further, the present inventors have identified that the properties of the final carbon heating element may be changed through the carbon heater manufacturing method , even though the same composition is used.
A carbon composite produced after the third heat treatment process is combined with a connector and a sealing tube to manufacture a carbon heater that is a final product, as shown in FIG. 5
An actual carbon heater includes a heating element 21 made of the carbon composite and a connector 24 for supporting the heating element 21 and supplying power from the outside. Also, the carbon heater further includes a tube 22 enclosing the heating element 21 and containing inert gas, a groove portion 23, a metal wire 25 for supplying electricity to the heating element 21 from the outside, a metal piece 26, an outer electrode 27, an outer connector 28, an outer terminal 29 and the like.
Hereinafter, the present invention will be described in more detail through various exemplary embodiments. The following exemplary embodiments are merely illustrative to more clearly describe the present invention, and the present invention is not limited thereto.

Example

A quaternary composition is prepared by adding 15 to 30 wt.% (hereinafter referred to as "%" or "wt.%") of a novolac resin as a binder and 0.1 to 10 % of graphite as a lubricant to the inorganic powder which is based on 50 to 75% of SiC and further includes SiO₂ as a specific resistance controlling agent, among the inorganic power components shown in Table 1 above.
The novolac resin used in this Example has preferably a number average molecular weight in the range of 1,000 to 10,000, more preferably a number of average molecular weight in the range of 3,000 to 7,000.
According to the method shown in FIG. 4, the prepared quaternary composition is mixed uniformly through the step of mixing raw materials, and extruded. Subsequently, the extruded composition is subjected to the stabilization heat treatment process and the first to third carbonization heat treatment processes, and followed by being processed into a final carbon heating element. And, the electrical properties of the final carbon heating element are evaluated.
FIGS. 6 and 7 show the electrical conductivity properties (FIG. 6) and the specific resistance and power properties (FIG. 7) of the composition including 59% of SiC, 15% of SiO₂, 23% of the binder resin and a 3% of the lubricant according to third carbonization heat treatment temperature.
As shown in FIG. 6, the electric conductivity of the carbon heating element increases in line with an increase in the third carbonization heat treatment temperature.
In FIG. 7, specific resistance, which is the inverse of the electrical conductivity, decreases in line with an increase in the third carbonization heat treatment temperature.
Meanwhile, property changes of the carbon heating element according to the third carbonization heat treatment temperature are not limited to the above described electrical properties.
FIG. 8 shows the thermal conductivity properties of the composition according to third carbonization heat treatment temperature.
As shown in FIG. 8, the carbon heating element shows a tendency that the thermal conductivity increases in line with an increase in the third carbonization heat treatment temperature, and then stabilizes or slightly decreases.
The changes in the electrical and thermal properties of the carbon heating element according to third carbonization heat treatment temperature shown in FIGS. 6 to 8 result from changes in the composition and microstructure of the carbon heating element composition according to third carbonization heat treatment temperature.
SiC, which is one of inorganic components used for manufacturing the carbon heating element, has crystal structures such as cubic known as β-SiC, hexagonal, and 170 types of rhombohedral. In general, the hexagonal and rhombohedral classes of SiC polytypes are collectively known as α-SiC (refer to Ceramist, ).
As shown in the state diagram of FIG. 9, SiC has a phase having different crystal structures over a temperature range of 1,000 to 2,700°C or more.
Such SiC greatly differs from SiO₂ in terms of melting point, thermal conductivity, and electrical properties, as shown in Table 1.
First, with respect to melting point, SiO₂, which has a melting point of approximately 1,600°C, may not exist in a solid state at a temperature higher than this melting point.
FIG. 10 shows an XRD pattern published by other researchers (Ceramics International 38 (2012) pp. 5223-5229).
FIG. 10 shows that the stability of SiC and SiO₂ varies according to temperature. More specifically, SiO₂ may no longer exist as a stable phase at 1,600°C or more. The intensity ratio of the diffraction peak of SiO₂ to SiC at 1,500°C is lower than the intensity ratio at 1,400°C, from which, it is apparent that the local decomposition of SiO₂ is already in progress at 1,500°C.
The experiment results of FIGS. 6 to 8 also correspond to the aforementioned experimental result.
In FIG. 8, the thermal conductivity of the carbon heating element increases in line with an increase in the temperature, and then decreases or becomes constant. In particular, the thermal conductivity increases continuously up to 1,600°C.
This is due to a difference in thermal conductivity between SiC and SiO₂, as shown in Table 1 above.
SiO₂ having low thermal conductivity becomes unstable as the third heat treatment temperature increases, and as a result, SiO₂ is coupled to carbon included in the composition and thus phase-transitioned to SiC having high thermal conductivity. As the ratio of SiC with high thermal conductivity increases, the macroscopic thermal conductivity of the carbon heating element increases.
Even though the third heat treatment temperature further increases, the phase transition is already completed at 1,600 °C. Thus, the thermal conductivity of the carbon heating element remains almost unchanged or slightly decreases even when heated to a higher temperature.
The carbon heating element is characterized by having a thermal conductivity of 1.6 W/m·K or more, for the following reason. When the thermal conductivity of the carbon heating element is lower than 1.6 W/m·K, heat is not properly dissipated in the vicinity of the terminal when a voltage is applied to the carbon heating element. As a result, excessive thermal stresses or thermal impacts are applied to the carbon heating element, and thus the carbon heating element having brittleness that is a property inherent in a ceramic material is likely to be destroyed by the thermal stresses or thermal impacts applied thereto.
FIG. 11 is a photograph showing a carbon heating element assembly in which a carbon heating element is destroyed after excessive thermal stresses are applied to the carbon heating element.
FIGS. 6 and 7 respectively show the increasing electrical conductivity of the carbon heating element and the decreasing specific resistance thereof in line with an increase in the third carbonization heat treatment temperature.
Changes in the electrical properties of the carbon heating element are also determined by the microstructure and components thereof.
As described above, SiO₂ is locally melted and coupled to carbon included in the composition in line with an increase in the third carbonization heat treatment temperature to be phase-transitioned to SiC.
Therefore, the ratio of SiO₂ having high specific resistance decreases in the carbon heating element, while the ratio of SiC having low specific resistance increases in the carbon heating element.
As a result, the specific resistance of the carbon heating element decreases (the electrical conductivity increases) in line with an increase in the third carbonization heat treatment temperature.
However, even when the third carbonization heat treatment temperature is higher than 1,700°C, the electric conductivity constantly increases and then is saturated, unlike the thermal conductivity.
This is due to a change in SiC constituting the carbon heating element.
FIG. 12 shows an X-ray diffraction (XRD) pattern of a carbon heating element, and a full width at half maximum (FWHM) of the XRD analysis result for measuring the degree of crystallization of SiC.
In the present invention, the XRD test is conducted using a D8 Advance model of Bruker. The XRD patterns are measured at a scan rate of 0.2 degree/sec under accelerating conditions of 60 kV and 80 mA by use of the Cu Kα wavelength. The XRD pattern is measured and analyzed using the software of Diffrac. Measurement Center/Diffrac. EVA.
As a result of the XRD test, the value of the FWHM decreases to 0.12 when the third carbonization heat treatment temperature increased to 1,800°C or more.
This means that, when the heat treatment temperature increases to 1,800 °C or more, the degree of crystallization of SiC generated by the third carbonization heat treatment as well as SiC existing in the initial composition of the carbon heating element increases.
In other words, various defects are reduced in SiC having a higher degree of crystalization, and as a result, the electrical conductivity increases (the specific resistance decreases).
It is apparent that the carbon heating element includes both β-SiC and α-SiC from the XRD analysis results of FIG. 12. In particular, α-SiC is preferable in that it has relatively high thermal conductivity and a large band gap in comparison to β-SiC.
Also, it is preferable that the carbon heating element has a FWHM value of 0.14 or more. When the FWHM value is less than 0.14, the degree of crystallization of SiC is excessively high, resulting in excessively high electrical conductivity and low specific resistance.
FIG. 13 shows SEM-EDS analysis results of the components before and after performing the third carbonization heat treatment on the composition including 56% of SiC, 18% of SiO₂, 23% of a binder resin, and 3% of a lubricant.
An amount of oxygen of the heating element composition before performing the heat treatment thereon is about 17%, but the oxygen amount of the composition after performing the heat treatment thereon is about 1%.
The oxygen that may exist in the carbon heating element is included in the composition of a starting material in the form of SiO₂, so that oxygen existing in the composition after performing the third carbonization heat treatment thereon will also exist as SiO₂.
But, as shown in FIG. 12, no peak for oxide is found in the XRD analysis results, unlike the EDS analysis results.
Therefore, it is expected that the oxygen of FIG. 13 is either partially decomposed SiO₂ or oxygen existing in a form other than SiO₂.
The amount of oxygen in the carbon heating element is preferably 2% or less, even considering an error range of EDS, for the following reason.
When the amount of oxygen exceeds 2%, the amount of remaining SiO₂ after performing the third carbonization heat treatment is excessively large, which results in low thermal conductivity and excessively high specific resistance.
FIG. 14 shows the surface temperature and output of a carbon heating element according to the specific resistance of the carbon heating element.
In general, the higher the specific resistance, the lower the power (energy) consumed when heating the carbon heating element to a certain temperature.
However, considering that the operating temperature at which the quartz tube may be used in the oven including the heater is 1,100°C, the carbon heating element has preferably a specific resistance of (11∼16) *10^-2Ωcm.
When the specific resistance of the carbon heating element is less than 11*10^-2Ωcm, the power for obtaining the heating temperature of a desired carbon heating element becomes too high, which is not preferable in terms of energy efficiency.
Conversely, when the specific resistance of the carbon heating element exceeds 16*10^-2Ωcm, the thermal conductivity is lowered along with the specific resistance, so that the carbon heating element may be easily destroyed.
FIG. 15 shows yields obtained by performing a third carbonization heat treatment on compositions including 56 to 62% of SiC, 12 to 18% of SiO₂, 23% of the binder resin, and 3% of the lubricant.
The yield is defined as a value obtained by dividing the weight of the carbon heating element, which is a final product, by the weight of the raw material before performing the third carbonization heat treatment, that is, the weight of the composition.
Referring to FIG. 15, it is apparent that, even when the third carbonization heat treatment temperature increases, the yield of the composition of the carbon heating element is not greatly changed. Rather, as the SiO₂ content increases, the yield tends to greatly decrease.
Such yield measurement results correspond to the changes in the composition and microstructure according to the third carbonization heat treatment of the carbon heating element, described above.
More specifically, as the third carbonization heat treatment temperature increases to 1,500°C or more, SiO₂ included in the carbon heating element is locally melted and coupled to carbon existing in the composition and thus phase-transitioned to SiC.
This means that relatively heavier SiO₂ is phase-transited to lighter SiC. As a result, the weight of the carbonized heating element is reduced and the yield is also reduced.
And, as the fraction of the phase transition increases, in other words, as the amount of SiO₂ in the carbon heating element composition increases, a reduction in weight resulting from the third carbonization heat treatment becomes larger, which leads to a further reduction in yield.
As shown in FIG. 16, a carbon heating element 110 may be provided in a solid bulk form having various shapes such as a rod shape having a circular cross-section (refer to FIG. 15 (a)), a rod shape having a rectangular cross-section (refer to FIG. 15 (b)), and a rod shape having a triangular cross-section (refer to FIG. 15 (c)).
Also, the carbon heating element may have a shape different from the aforementioned shapes.
FIG. 17 shows one shape of a carbon heating element 210 included in the carbon heater.
That is, the carbon heating element 210 shown in FIG. 17 is formed in a tube shape having a central aperture 210a.
Here, the size of the central aperture 210a or the ratio of the central aperture 210a to the entire cross-sectional area of the carbon heating element 210 may be changed in various ways, and is not limited to the illustrated shape.
FIG. 18 shows another shape of the carbon heating element 210 included in the carbon heater.
That is, the carbon heating element 210 shown in FIG. 18 is formed in a tube shape having the central aperture 210a. Unlike the carbon heating element 210 shown in FIG. 17, the carbon heating element 210 of FIG. 18 has a shape in which a portion of the tube is cut such that a circular arc is provided with an opening 210b.
The carbon heating elements 210 shown in FIGS. 17 and 18 are different from each other in terms of whether or not there is the opening 210b (refer to FIG. 10), but are similar to each other in that both have the central aperture 210a.

Claims

A heating element for a carbon heater, comprising carbon (C) and silicon carbide (SiC), wherein the heating element has a thermal conductivity of 1.6 W/m·K or more.
The heating element according to claim 1, wherein the SiC comprises β-SiC and α-SiC.
The heating element according to claim 1 or 2, wherein a degree of crystallization of the silicon carbide in the heating element has a full width at half maximum (FWHM) value of 0.14 as a result of analyzing an X-ray diffraction.
The heating element according to any of claims 1 to 3, wherein the heating element comprises silicon oxide (SiO₂).
The heating element according to any of claims 1 to 4, wherein a total amount of oxygen (O₂) in the heating element is 2 wt.% or less.
The heating element according to any of claims 1 to 5, wherein a specific resistance of the heating element is (11-16) *10^-2Ωcm.
The heating element according to any of claims 1 to 6, wherein the heating element (110) has a rod shape.
The heating element according to any of claims 1 to 6, wherein the heating element (210) has a hollow space.
A method for manufacturing a carbon heater including a heating element, comprising:
a mixing process to mix component materials of the heating element;

a thermal extrusion process to form a shape of the heating element by extruding and injecting the mixed materials;

a stabilization heat treatment process to form a coupling structure of carbon and oxygen of the binder within the heating element; and

a carbonization heat treatment process to out-gas a volatile component out of the components constituting the composition of the heating element and carbonize the remaining components.
The method according to claim 9, wherein the component materials comprises:
a base material determining a specific resistance of a heating element, the base material including SiC;

a specific resistance controlling agent for controlling a specific resistance of the heating element, the specific resistance controlling agent including SiO₂;

a lubricant including graphite; and

a binder for mechanical coupling between inorganic powers, the binder including a novolac resin.
The method according to any of claims 9 and 10, wherein the extrusion process is performed at a speed of about 60rpm at 100 to 200°C.
The method according to any of claims 9 to 11, wherein the stabilization heat treatment process is performed at 270 to 320°C for 10 minutes to 2 hours.
The method according to any of claims 9 to 12, wherein the carbonization heat treatment step comprises a first carbonization heat treatment process for out-gassing at 600 to 1,000°C for 10 minutes to 2 hours.
The method according to any of claims 9 to 13, wherein the carbonization heat treatment process comprises a second carbonization heat treatment process and/or a third carbonization heat treatment process which is different from the second carbonization heat treatment process.
The method according to claim 14, wherein the second carbonization heat treatment process is performed at 1,200 to 1,400°C for 10 minutes to 4 hours, and the third carbonization heat treatment process is performed at 1,500 to 1,700°C for 10 minutes to 4 hours.